This invention relates to apparatus for testing a circuit board.
It is conventional to test a populated circuit board, i.e. a circuit board comprising a support member and an electronic component mounted on the support member, by connecting individual probes to the leads of the component, and using a test instrument to observe the waveforms existing at those leads. The probes are connected to the leads of the component by mechanically gripping the leads. This technique is not applicable to a component that is surface mounted on the support member, because the component does not have leads that can be readily gripped, and in any event the leads are too close together to be gripped without risk of causing a short circuit between the leads.
The co-pending application referred to above describes a probe that comprises a flexible sheet of dielectric material, contact pads on a first surface of the sheet, and conductor runs connecting the contact pads to terminals of the probe. The pattern of contact pads on the first surface of the sheet corresponds to the pattern of test points on a circuit board to be tested. In use of the probe, the flexible sheet is positioned with its first surface in confronting relationship with the circuit board and the pattern of contact pads in registration with the pattern of test points. Pressure is applied to the flexible sheet, whereby the contact pads are brought into contact with the test points respectively.
The probe described in the co-pending application referred to avoids the problems discussed above in testing a populated circuit board. However, this probe is nevertheless subject to disadvantage, in that it is not suitable for testing a circuit board when the test point is close to a component that is substantially higher than the test point, since engagement of the flexible sheet with the component interferes with reliable contact being established between the contact pad of the probe and the test point in the board.
Co-pending application Ser. No. 07/338,786 filed Apr. 14, 1989 describes a probe that is suitable for testing a populated circuit board and comprises a sheet of dielectric material having an opening therein sized and shaped to receive an electronic component mounted on the support member of the circuit board. The probe also comprises a contact pad exposed at a first main face of the sheet, and a conductor run connecting the contact pad to a terminal of the probe. The probe is positioned with its first main face in confronting relationship with the major surface of the support member, with the electronic component received in the opening in the sheet of dielectric material, and pressure is applied to the probe, whereby the contact pad contacts the test point. This probe avoids the above-mentioned disadvantage of the probe described in the first co-pending application referred to above, but is nevertheless subject to disadvantage in that it is not well suited for probing test points that are close together, because there is a danger that the contact pad will bridge two adjacent test points and cause a short circuit.
In the probe described in the second co-pending application referred to above, the opening in the sheet of dielectric material assists in locating the probe relative to the support member of the circuit board. The efficiency with which the hole locates the probe depends in part upon the shape of the electronic component. If the shape of the component is such that, with normal manufacturing, the probe is able to shift slightly relative to the electronic component, the risk of the test points' being short circuited is substantially increased.